A Guide to Better Impressions

Retraction:

Good impressions start with good retraction, but often the process is rushed and the impression is resultantly compromised. Most trained prosthodontists agree that two to three minutes is insufficient to achieve good tissue deflection. Consequently, most prosthodontists pack retraction cord for at least five minutes to achieve good tissue memory. This is one of the main secrets in achieving a good impression. Let's discuss two alternative methods. 

Single Cord Technique: 

This technique is most common, but often insufficient. The single cord retraction technique is best used when preparing margins at, or above tissue. If gingival tissues are healthy and no bleeding occurs when the cord is packed, this is usually an accepted method. 

Double Cord Technique: 

The double cord retraction technique is best used when sub-gingival margins are required, and/or if the tissue health is less than ideal. Double cord technique is not simply inserting two cords. The technique sequence listed below will give the best result.

• Remove enough coronal structure to gain easy access to the interproximal areas. Try to avoid preparing areas close to the gingival tissue. Gently insert an extra thin cord such as Deknatel suture or # I retraction cord (impregnated with an astringent of your choice) into the sulcus. 

• The Deknatel or # I cord will provide a slight tissue deflection allowing more access but more importantly, it will serve as a depth gauge to help avoid cutting epithelial tissue making moisture/blood management much easier during the impression process. 

• Finish the rest of the impression including the gingival margins with the Deknatel in place. Then pack a braided cord impregnated with astringent to achieve adequate tissue displacement. If the sulcus is deep and the braided cord goes below the tissue and is not clearly visible, place another layer of cord. Cord placed below tissue will simply train the upper tissue to curl, defeating the process of tissue deflection. Furthermore, try to use the largest braided cord possible to achieve the greatest tissue deflection. 

• After a minimum of five minutes, remove the upper braided cord, (leaving the suture cord in place) and proceed with the impression. 

Impressioning 

Tray / Injection Sequence:

Impression material working time defects are extremely common. Regardless of which impression system is used, it is advisable to load the tray material, prior to beginning the injection process. Tray materials loaded at room temperature will have substantially more working time than materials used in the mouth at 98 degrees. The incident that we want to avoid is waiting for the tray to be completed while material is sitting on the preparation. 

Syringe Technique: 

Most Dentists were taught to start the injection process on the lingual/distal aspect of the preparation and circle to preparation up to the occlusal/incisal aspect. Often "V" shaped voids develop at the start point. To reduce all voids, a slightly modified syringe technique is as follows; 

With the syringe tip resting against the preparation begin the syringing process. While syringing, use a slight stirring motion by gently wiggling the tip back and forth and up and down while expressing material. Use this stirring movement in the intexproximal areas as well. This process will help the impression material relax, better wetting the tooth surface and helping the material to blend with itself This stirring method is much preferred to using the air syringe to blow the first layer and then reapplying additional material. Using the air method robs the operator of valuable working time. 

Tray Insertion: 

Why is it that trays are inserted at the speed of light? Fewer defects occur if an additional two seconds are taken to properly align the tray and slowly seat to place. This will prevent drags, pulls and distortions caused by tray realignment once the tray has been seated. If there is any tooth-to-tray contact using a plastic double-bite tray, please retake the impression. 

Additional Tips: 

• Most impression materials allow only one minute of intra-oral working time. We advise that a cartridge of injectable material be stored in a refrigerator and be reserved for cases involving three or more units. The lower temperature will extend the working time by as much as 50%. Consequently, set time is also retarded. Thus extend the normal intra-oral set time by at least 50 percent. This refrigerated material is also useful on hot summer days when the office gets warmer than normal. 

• It is one of those days where we are on our second or third impression and the schedule is in chaos. If an impression is removed to soon it may look perfect, but distortion has undoubtedly occurred because the material did not achieve full physical properties and it did not rebound to natural position. This is the primary reason for tight fitting crowns. 

• If tooth to tray contact occurs, it is almost certain that distortion will occur. 

• Never exceed two units when using a double bite tray. When two or more units are involved, a full arch impression is advised. 

• A bite registration including the preparation site and at least two of the adjacent teeth is invaluable for proper model articulation. This separate registration is advised even with double bite impressions.